1. Field
One embodiment of the present invention relates to a disk drive. More particularly, the invention relates to a technique of controlling the offset in a disk drive.
2. Description of the Related Art
Most disk drives, a representative example of which is the hard disk drive has a disk and a magnetic head. The disk is a magnetic recording medium. The magnetic head is configured to record and reproduce data in and from the disk. The magnetic head is mounted on a rotary-type actuator The magnetic head can be moved in the radial direction of the disk and positioned at a target track (or cylinder) provided on the disk.
The magnetic head has a read head and a write head, which are mounted on a slider (head main body) and spaced apart from each other. The read head is, for example, a GMR element, and is configured to read data recorded in the disk. The write head is configured to write data in the disk. Depending on the position the magnetic head assumes in the radial direction of the disk, an offset (positional deviation) develops between the track loci of the read head and the write head.
In order to move the magnetic head to a target position over the disk, an offset control is performed on both the read head and the write head to adjust the positions of the read head and write head in accordance with the offset. The offset control is performed based on the offset value that changes with the position the magnetic head takes in the radial direction of the disk. Note that the offset value remains the same as long as the magnetic head remains in the same track.
In the disk drive, a phenomenon called “disk runout” may develop if the spindle motor is secured at a wrong position. Once a disk runout has developed, the servo track deviates from the circular locus with respect to the rotation center of the disk. This results in a servo-track runout. Consequently, no accurate offset control can be performed on the magnetic head in the same track, because the offset value remains unchanged while the disk is rotating 360°.
To perform accurate offset control, a technique called “dynamic offset control (DOC) has been proposed (see, for example, Jpn. Pat. Appln. KOKAI Publications Nos. 2005-216378 and 2007-172733). The dynamic offset control is performed by changing the offset value in accordance with the disk runout value. Jpn. Pat. Appln. KOKAI Publication No. 2005-216378 discloses an offset control that uses first and second offset values. The first offset value changes with the position the head takes in the track in the radial direction of the disk. The second offset value is calculated, changing as the disk rotates 360°. On the other hand, Jpn. Pat. Appln KOKAI Publication No. 2007-172733 discloses a method that uses the first offset value identical to that described above, and monitors the second offset value that changes as the disk rotates 360°, directly on the basis of the error rate of data.
With regard to the dynamic offset control (DOC), a technique of measuring the disk runout has been proposed (see, for example, Jpn. Pat. Appln KOKAI Publication No. 11-126444 and Japanese Patent No. 3198490). More specifically, Jpn. Pat. Appln KOKAI Publication No. 11-126444 discloses a method in which the actuator holding a magnetic head is set at a specific position, the cylinder address information for one rotation of the disk from the servo sector, and the servo-track runout is measured from the cylinder address information. Japanese Patent No. 3198490 discloses a method of inferring a servo-track runout from a change in the time intervals of lock marks for one rotation of the disk.
As described above, due to the disk runout, the servo track that serves as the reference position for head positioning deviates from the circular locus with respect to the rotation center of the disk (resulting in a servo-track runout). This means that the read head positioned at the servo track changes in the radial direction of the disk as the disk rotates 360°. Therefore, the offset between the read head and the write head must be changed as the disk rotates once.
The servo-track runout occurs, mainly because the disk runout that develops when a servo-track writer records servo data in the disk, thereby forming servo tracks on the disk. Further, another disk runout develops when the disk with the servo data recorded on it is incorporated into the disk drive. The two disk runouts combine, resulting in a larger disk runout.
The above-identified prior-art publications propose a method of changing the offset value as the disk rotates 360°. In this regard, a method is employed, in which the offset value is calculated or inferred from the servo-track runout information and the sizes of the parts and mechanisms constituting the disk drive. However, the offset value calculated by this method is far from accurate. It is difficult for this method to provide an accurate setoff value, particularly because the sizes of the parts and mechanisms greatly vary from one disk drive to another. Moreover, when the disk drive receives an impact from outside, a disk shift occurs, inevitably changing the disk runout. It is therefore desirable to measure the offset value every time the magnetic head is loaded above the disk. Thus, the offset value must be measured frequently, requiring some time each time. This ultimately impairs the data-access ability of the disk drive.